There has been growing recognition of high frequency with which Americans utilize complimentary and alternative health care providers, and that annually about 10% of the population is treated by a chiropractor. While chiropractors employ several types of treatment, spinal manipulation is the most important and unique of these therapies. Over 90% of spinal manipulation performed in the United States is done by chiropractors with most of the remainder performed by osteopathic physicians. The annual number of chiropractic visits in the United States has been estimated at approximately 100 per 100 person years. There are several difficulties in researching the effects of spinal manipulation. Most of the conditions that are treated do not have a well defined pathophysiology. Therefore it is not possible to study treatment by evaluating the effects of treatment on physiological variables. The lack of proven pathophysiology also leads to heterogeneous study populations with large variability and response to treatment. This makes it necessary to use large experimental populations in order to detect any effects of treatment. There is heavy reliance on subjective outcome measures in neck and back pain research making these clinical trials particularly sensitive to placebo or "nonspecific" effects. The use of blinding subjects is the accepted method of dealing with these issues, but blinding is particularly difficult to achieve when investigating physical interventions, particularly those where the therapist can't be blinded as to whether she or he is delivering an actual or sham treatment.